Optical encoding techniques are used to indicate the position and other attributes of an object, such as an ink-jet print head used in an ink-jet printer, a thermal print head, a rotary shaft, or a paper feeder. Examples of attributes include linear position of the object, the rotational position of the object, and the speed of the object. Conventional printers often use an optical encoder system with and optical encoder medium.
A typical optical encoder system has an emitter section that directs light toward a detector section that includes one or more photodiodes, signal processing circuitry, and other circuitry. In some cases, the light from a light-emitting diode in the emitter section is collimated into a parallel beam by a lens. Conventional optical encoder media, such as optical code wheels or optical code strips has a pattern of transparent and opaque sections, also known as spaces and bars. As the emitter/detector section moves relative to the optical encoder medium, the light beam is interrupted by the code strip or code wheel. The detector section transforms the interrupted light into electrical signals that indicate the position of the emitter/detector section.
Unfortunately, an optical encoder system can fail if it becomes contaminated, such as with ink mist or dust from paper or the environment. Such contamination can scratch the lens and/or optical encoder medium (e.g. optical code strip), which can cause a catastrophic failure of the optical encoder system. A number of solutions have been proposed to avoid such failures in optical encoders, including: higher sensitivity photo detectors, more photo detectors, increased light output, different light wavelengths, and adding contamination shields and filters. However, optical encoder systems remain vulnerable to contamination and damage from contamination. Therefore, encoder systems avoiding the above-mentioned problems are desired.